1. Field of the Invention
This invention relates to injection molding and particularly to injection molding of molding material into molds that may contain fibers and/or cores, and apparatus and method therefore.
2. Description of Related Art
Currently available manufacturing techniques can produce composite materials that are far stronger than the best specialty steels. Within a composite material the long, oriented fibers absorb the tensile stresses; the matrix material serves merely to orient and hold the fibers in place. Glass fiber, for example, can be manufactured that will withstand a tensile stress in excess of 700,000 pounds per square inch. This is a far higher stress than any epoxy--or steel, for that matter--could withstand. Unfortunately, manufacturing procedures now used to produce parts with long, oriented fibers, such as glass reinforced epoxy, are expensive and limited to a narrow spectrum of shapes and sizes.
On the other hand, injection molding techniques offer a versatile and inexpensive means of producing high quality parts with fine details and intricate shapes. However, current methods of production do not result in moldings that are strong for a given weight of material. The molding material used is necessarily homogeneous in nature and the stress-absorbing properties of available molding materials are limited. The strength of an injection molding may be increased by mixing short strands of chopped fiber into the molding material, thus producing a composite material. But if chopped strands are included that are long enough, and numerous enough, to significantly affect the strength of the molded part, they will not inject well or at all. Furthermore, the orientation of fiber strands in an injected composite material is random rather than aligned along stress lines. In composites with a homogeneous distribution of short fibers, the bonding and tensile strength of the matrix material becomes the stress limiting factor, not the fibers themselves. Fibers may also be highly eroding and can damage injection machines.
A mold may be pre-loaded with long, oriented fibers prior to injection of molding material. The primary problem molders face when attempting to inject matrix material into a mold that has been pre-loaded with fibers, and possibly core structure such as foam or inserts intended to take a screw or the like, is the fact that air (and sometimes other gasses) tends to become trapped and to displace the matrix material. This problem is exacerbated by the fact that the fibers used must be numerous to be effective and thus provide many small spaces between strands that act as gas traps. Several approaches have been attempted to solve this problem, some of which have been successful. For example, compression molding techniques, which wet the fibers with matrix material prior to placement in the mold and then squeeze excess material out of gaps between mold halves under high pressure, are effective with simple moldings but a complex mold will trap air pockets that ruin the final product. Pultrusion molding is effective but limited to simple shapes with an extrudable cross section. Both compression molding and pultrusion molding are relatively inexpensive.
Vacuum venting will aid injection of matrix material into a mold containing fibers and cores. The vacuum reduces the quantity of gasses in the mold that are later compressed by the matrix material into small, entrapped bubbles. Vacuum venting is not, however, currently used with the purpose of removing the gasses from the molding, but to speed the injection process by reducing internal mold pressure. The quality of the final product made with vacuum venting is still highly dependant on the shape of the mold, the placement of the vent holes, and the location of injection points.
A vacuum will reduce the absolute number of gas molecules remaining in a mold as the gas pressure is reduced, and this fact suggests an extension and improvement to injection molding practice. First remove the gasses from the mold and any fibers it contains using a vacuum, and then inject the matrix material. This approach to injection molding is no panacea; the use of a vacuum to remove air and gasses from a mold has some disadvantages, such as an increase in equipment costs and process time. But the use of a vacuum when molding also has some unexpected advantages, namely, that given sufficient time before injection of the matrix material a vacuum will remove residual gasses from cracks and crevasses in fiber, tend to remove absorbed and chemisorbed gasses from surfaces of fibers, remove water from the system and generally clean the fiber before the matrix material is introduced (Robinson, Norman W., 1968, The Physical Principles of Ultra-high Vacuum Systems and Equipment. Chapman and Hall, Ltd. 11 New Fetter Lane, London EC4, chapters 6 and 7). Cleaning the fibers improves bonding with the matrix material, and therefore the ultimate strength of the final product.
The process of gas extraction from a mold and its contents with a vacuum takes an extended period of time. As the gas pressure within a mold is reduced, the molecular density thins to a point where the mean free path length of molecular travel is longer than the distance between internal surfaces. At this point molecular flow is a function of random elastic collisions of individual molecules with internal surfaces, which may or may not lead to a particular molecule exiting the mold. Eventually, given a low enough pressure external to the mold to assure minimal molecular back flow, most gas molecules will exit. However, absorbed water on internal surfaces of the mold and contents will vaporize as the gas pressure is lowered, causing spaces vacated by other gas molecules to be occupied by water vapor molecules which then must be removed. Chemisorbed atoms that are maintained in an equilibrium surface concentration on internal surfaces by molecular gas pressure also occupy space vacated by other molecules when released and must, in turn, be removed. The effect of volatilizing adsorbed and chemisorbed water and gasses is to increase the internal gas concentration of the mold and contents far above what would be expected with a simple calculation of the space involved. This "desorption load" is not, however, prohibitive in a properly designed production apparatus. Experience with molding a very complex, 76% glass fiber, roughly one-quarter liter volume component, with the relative humidity of about 75%, has shown that adequate gas removal can be accomplished by pumping down to 6.times.10.sup.-4 mbar for about forty-five minutes through an injection port roughly two millimeters in diameter prior to epoxy injection. Adsorbed water from high humidity appears to be the main source of desorption load and can be significantly alleviated by lowering relative humidity in the working area.
A vacuum first, injection second process has no need for an array of vents and risers. Complex and intricate shapes in the part will not trap gasses because there is little gas to trap. Injection of matrix material can be from any location on the mold because there is nothing to displace and no need to consider the flow characteristics of the matrix material in terms of displacing air toward a vent. In the case of a very complex part, or perhaps a very large part, the manufacturer is not rushed by the curing time of the matrix material he uses because the fiber lay up is done dry. Furthermore, conventional injection molding processes that use high pressure usually require expensive steel molds. The method disclosed in this specification can use much simpler molds made of plastic or the like because all pressures are relatively modest.
How much gas must be removed prior to injection of matrix material if a mold is tightly stuffed with fibers? Assume that the final volume of a molding is one liter and 80% of this volume is taken up by fiber. To maximize the strength of a product the ratio of fiber to matrix material should be high, and for glass/epoxy a ratio of 80% glass to 20% epoxy has very good properties. In a one liter mold, then, two hundred milliliters of gas (air, contaminants and water) would remain in the mold after placing 800 milliliters of fiber in it. The gas must be removed, either with venting or with vacuum, to produce a successful molding.
Dual stage vacuum pumps are commercially available that can reduce the gas pressure in the mold to less than 6.times.10.sup.-4 mbar. Much higher vacuums can be achieved with additional equipment. A vacuum of 6.times.10.sup.-4 mbar is roughly the point at which oil vapor pressure in the pump equals the vacuum pressure, thus imposing a lower vacuum limit on simple "roughing" pumps. We may now ask the question: If the pressure in the mold is first reduced to 6.times.10.sup.-4 mbar and matrix material is then injected until the volume of gas is reduced by displacement until it again comes up to atmospheric pressure, what would the residual gas volume be? The ideal gas laws may be stated as: ##EQU1## substituting: ##EQU2## One hundred eighteen nanoliters of gas would remain in a one liter molding. Or, in other words, a bubble of gas roughly 0.6 millimeter in diameter. This is a very small bubble, assuming all the gas remains in one place. Multiple bubbles would be proportionately smaller.
We do not, however, need to accept a bubble even this small. If a turbomolecular pump is added to a two-stage roughing pump, five additional orders of magnitude of vacuum can be achieved--thus reducing the residual gas in a molding proportionately. That is, the residual gas in a one-litter molding would now have a volume of 1.18 picoliter. This is a spherical bubble roughly 6 nanometers in diameter (that is, a diameter roughly 1% of one wavelength of red light). A further reduction in gas bubble size can be made if the matrix material is now injected with a pressure greater than atmospheric. Or, if vacuum in the mold is maintained at a more or less constant level during injection of the matrix material by venting to vacuum during injection, the volume of residual gas is reduced yet again. The calculations above are made to demonstrate the point that there is almost always a bubble size that is small enough to be acceptable as a residual imperfection in a molding, and this may be achieved in a mold of any shape. It must be realized that these calculations are based on the well known ideal gas laws, and as such are only an approximation, albeit a close enough one for our purposes here. Of course the engineer considering these issues must also consider how a vacuum will affect the matrix material used and factor in the additional costs involved with using vacuum equipment. Preliminary production of parts using the vacuum first, injection second process disclosed in this specification have shown no effect on epoxy of a 6.times.10.sup.-4 mbar vacuum, which is sufficient for most purposes.
Very large and complex structures can be molded using the method disclosed here. When attempting to produce a large complex structure with conventional techniques the matrix material may be curing as work progresses. Laying up the product then becomes a race against the chemistry of the matrix material--and that chemistry imposes a natural limit on size. If the part being made is too complex to make in one operation then there is a problem of bonding the next stage of construction with the previous one, which has hardened and formed a difficult surface to bond to. With the vacuum-injection process disclosed here, days or even years can be taken to place the fiber in the mold as desired, in as complex and large a shape as is needed. When a molding becomes large enough to make the degassing process prohibitively long, multiple vacuum and injection ports can be used to reduce the effective volume served by each. Injection of the matrix material is the last step in the molding process. The injection process occurs in a short time so there is no need to bond surfaces together. The entire structure is injected in one operation.
The method and apparatus disclosed in this specification allow an engineer to approach in practice the theoretical maximum strength per unit weight for many composite systems and does so in a practical, manufacturable fashion.